FIG. 1 shows a long term evolution (LTE) user-plane protocol stack. The user-plane includes a packet data convergence protocol (PDCP) entity, a radio link control (RLC) entity and a medium access control (MAC) entity.
The main functions of the RLC entity include transfer of upper layer protocol data units (PDUs) supporting acknowledged mode (AM), unacknowledged mode (UM), and transparent mode (TM) data transfer, error correction through automatic repeat request (ARQ), segmentation according to the size of the transport block (TB), re-segmentation of RLC PDUs that need to be retransmitted, concatenation of RLC service data units (SDUs) for the same radio bearer, in-sequence delivery of upper layer PDUs except at handover in the uplink, duplicate detection, protocol error detection and recovery, flow control between an evolved Node B (eNB) and a wireless transmit/receive unit (WTRU), SDU discard, RLC reset, or the like.
An RLC SDU may be segmented if the RLC SDU does not fit entirely into the TB. The RLC SDU may be segmented into variable sized RLC PDUs. If a retransmitted PDU does not fit entirely into the new TB used for retransmission, the RLC PDU may be re-segmented. The number of re-segmentation is not limited.
According to 3GPP TS 36.322 V1.2.0, the evolved universal terrestrial radio access network (E-UTRAN) RLC entity may perform SDU discard based on a notification from the PDCP entity above it, as opposed to having the RLC entity have its own SDU timer-based discard mechanism like in the Release 6 UTRAN RLC. According to 3GPP TS 36.322 V1.2.0, in E-UTRAN, a new re-segmentation functionality will be supported in addition to supporting segmentation. In addition to segmentation and reassembly of RLC SDUs, re-segmentation and reassembly of RLC PDUs will be supported (at least for AM data transfer).
FIG. 2 shows segmentation and re-segmentation of an RLC PDU. An RLC SDU may be segmented into RLC PDUs. Each RLC PDU is identified by a sequence number (SN) that is assigned on a per-RLC PDU basis, (i.e., a PDU SN). The RLC PDU SN is included in the RLC header. Segmentation of the RLC SDU into RLC PDUs may be performed once and an RLC SDU may not be segmented again. Instead, re-segmentation may be performed on RLC PDUs.
An RLC PDU may be segmented into PDU segments, (i.e., sub-segments). As shown in FIG. 2, a sub-segment may be identified by two parameters: a segment offset (SO) and a segment length (SL). The SO indicates the starting position of the segment within the original RLC PDU, (e.g., in bytes), and the SL indicates the length (size) of the segment, (e.g., in bytes).
RLC PDU re-segmentation may be performed multiple times without a limit according to 3GPP TS 36.322. FIGS. 3(A) and 3(B) show two RLC PDU re-segmentations (i.e., two occurrences of re-segmentation). In FIG. 3(A), the second sub-segment is larger than the first sub-segment. This might occur when the TB size selected by the lower layer is larger than the size of the first sub-segment that needs to be retransmitted. In FIG. 3(B), the second sub-segment is smaller than the first sub-segment. This might occur when the TB size selected by the lower layer is smaller than the size of the first sub-segment that needs to be retransmitted.
In the UTRAN, the RLC AM mode performs ARQ retransmissions. One of the parameters for the ARQ mechanism is the maximum number of transmissions, MaxDAT. MaxDAT represents the upper limit for state variable VT(DAT). The state variable VT(DAT) counts the number of times that an AMD PDU has been scheduled to be transmitted. There shall be one VT(DAT) for each RLC AMD PDU and each shall be incremented by one every time the corresponding AMD PDU is scheduled to be transmitted. The initial value of VT(DAT) is 0. When VT(DAT) equals MaxDAT, either an RLC reset procedure or an SDU discard procedure shall be initiated according to the configuration by the upper layer.
In the E-UTRAN, it has been agreed to support the RLC SDU discard functionality in the PDCP entity, (e.g., timer-based discard). The PDCP entity notifies the RLC entity of a discarded SDU/PDU so that the RLC entity discards it from its buffers.
However, there is a need for an RLC PDU (and in turn RLC SDU) discard criteria to be evaluated and implemented locally in the RLC entity itself. Such mechanism may be used to avoid protocol deadlock conditions, (e.g., indefinite retransmissions), and/or to trigger other procedures, such as RLC reset or re-establishment and RLC move receive window (MRW), and/or to support better quality of service (QoS). The UTRAN RLC provides an MRW procedure which is a signal sent by the sending RLC entity to request the receiving RLC entity to move its reception window, and optionally to indicate the set of discarded RLC SDUs as a result of an RLC SDU discard in the sending RLC entity.
In the UTRAN, the MaxDAT parameter was used to discard RLC PDUs based on the number of PDU (re)transmissions, and in turn trigger either an RLC reset procedure or an RLC MRW procedure. For the E-UTRAN, the straightforward MaxDAT parameter based on the number of PDU (re)transmissions may not be used due to the introduction of the new functionality of re-segmentation in the E-UTRAN, which makes the straightforward counting of PDU (re)transmissions inappropriate as a discard criterion for RLC PDUs.
Therefore, it would be desirable to provide methods and criteria to discard an RLC PDU, and/or in turn the corresponding RLC SDU(s), and/or to trigger other procedures, such as RLC reset or re-establishment.